The term "image recording" is conventionally taken to mean a process which produces a spatial pattern of optical absorption in the recording medium. Photographic processes are well known examples of this type of process.
In a broader sense, however, the word "image" means a spatial variation of the optical properties of a sample in such a way as to cause a desired modification of a beam of light passing through the sample. Refractive index images in general and holograms in particular, which modulate the phase, rather than the amplitude, of the beam passing through them are usually referred to as phase holograms. Phase holographic image recording systems produce a spatial pattern of varying refractive index rather than optical absorption in the recording medium and, thus, can modulate a beam of light without absorption.
This type of refractive index image also includes a number of optical elements or devices which superficially bear little resemblance to absorption images. Examples are holographic lenses, gratings, mirrors, and optical waveguides.
Holography is a form of optical information storage. The general principles are described in a number of references, e.g., "Photography by Laser" by E. N. Leith and J. Upatnieks in SCIENTIFIC AMERICAN 212, No. 6,24-35 (June, 1965). In brief, the object to be photographed or imaged is illuminated with coherent light, e.g., from a laser, and a light sensitive recording medium, e.g., a photographic plate, is positioned so as to receive light reflected from the object. Each point on the object reflects light to the entire recording medium, and each point on the medium receives light from the entire object. This beam of reflected light is known as the object beam. At the same time, a portion of the coherent light is directed by a mirror directly to the medium, bypassing the object. This beam is known as the reference beam. What is recorded on the recording medium is the interference pattern that results from the interaction of the reference beam and the object beam impinging on the medium. When the processed recording medium is subsequently illuminated and observed appropriately, the light from the illuminating source is diffracted by the hologram to reproduce the wave-front that originally reached the medium from the object, so that the hologram resembles a window through which the virtual image of the object is observed in full three-dimensional form, complete with parallax.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides, so that they are traveling in approximately opposite directions, are known as reflection holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, planes parallel to the plane of the recording medium. When the hologram is played back these fringes act as mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection rather than in transmission. Since the wavelength sensitivity of this type of hologram is very high, white light may be used for reconstruction.
Reflection holograms may be produced by an in-line or on-axis method wherein the beam of coherent radiation is projected through the recording medium onto an object therebehind. In this instance, the reflected object beam returns and intersects with the projected beam in the plane of the recording medium to form fringes substantially parallel to the plane of the medium. Reflection holograms also may be produced by an off-axis method wherein a reference beam is projected on one side of the recording medium and an object beam is projected on the reverse side of the medium. In this instance the object beam is formed by illuminating the object with coherent radiation which does not pass through the recording medium. Rather, the original beam of coherent radiation is split into two portions, one portion being projected on the medium and the other portion being projected on the object behind the medium. Reflection holograms produced by an off-axis process are disclosed in U.S. Pat. No. 3,532,406 to Hartman.
A holographic mirror is the simplest possible reflection hologram. It is the hologram of two coherent plane waves intersecting in a recording medium from substantially opposite directions. It can be created by splitting a single laser beam and recombining the beams at the recording medium, or the unsplit laser beam can be projected through the medium onto a plane mirror therebehind. A set of uniformly spaced fringes with approximately sinusoidal distribution is formed which are oriented parallel to the bisector of the obtuse angle between the two projected beams. If the obtuse angle is 180.degree. and the projected beams are normal to the plane of the medium, the fringes will be parallel to the plane of the medium. If the obtuse angle is less than 180.degree. or if both beams are not normal to the plane of the medium, reflective fringes will be formed which will be slanted at an acute angle relative to the plane of the medium. The holographic mirror can be characterized by its reflection efficiency, that is the percent of incident radiation which is reflected, refractive index modulation, and by the spectral bandwidth and character of the reflected radiation.
The substantially horizontal fringes which form reflection holograms are much more difficult to record than the perpendicular fringes which form transmission holograms for two reasons. The first reason is the need for higher resolution, i.e., the need for more fringes per unit length, and thus a smaller fringe spacing. Horizontal reflection holograms require about 3.times. to 6.times. more fringes per unit length than do transmission holograms. The second reason is the sensitivity of horizontal fringes to shrinkage of the recording medium. Any shrinkage of the recording medium during exposure will tend to wash out the fringes and, if severe, will prevent a hologram from being formed. This is in contrast to the transmission hologram case, where shrinkage has little or no effect if the fringes are perpendicular to the plane of the medium, and produces only relatively minor image distortion if the transmission fringes are slanted more than 45.degree. from the plane of the medium.
A variety of materials have been used to record volume holograms. Among the more important are: silver halide emulsions, hardened dichromated gelatin, photorefractives, ferroelectric crystals, photopolymers, photochromics and photodichromics. Characteristics of these materials are given in Volume Holography and Volume Gratings. Academic Press, New York, 1981 Chapter 10, pp. 254-304 by L. Solymar and D. J. Cook.
Dichromated gelatin is currently the material of choice for making reflection holograms due to its high diffraction efficiency, wide bandwidth response, and high values of refractive index modulation (i.e., dichromated gelatin exhibits low "background noise"). However, dichromated gelatin has poor shelf life and requires wet processing after the material has been imaged to contain a reflection hologram. Due to its poor shelf life, the material must be freshly prepared shortly before imaging or prehardened gelatin must be used, which reduces image efficiency. Wet processing introduces an additional step in preparation of the hologram, and causes dimensional changes in the material as it swells, then shrinks, during processing. These dimensional changes affect spacing of the interference fringes. Thus, it is difficult and time consuming to reproducibly make high quality reflection holograms with dichromated gelatin.
Substantially solid, photopolymer films have heretofore been proposed for use in making holograms. U.S. Pat. No. 3,658,526 to Haugh, for instance, discloses preparation of stable, high resolution holograms from solid, photopolymerizable films by a single step process wherein a permanent refractive index image is obtained by a single exposure to a coherent light source bearing holographic information. The holographic image thus formed is not destroyed by subsequent uniform exposure to light, but rather is fixed or enhanced.
Despite the many advantages of the materials proposed by Haugh, they offer only limited viewing response to visible radiation and application has been limited to transmission holograms where the holographic image is viewed by diffraction patterns created in light transmitted through the imaged material. Moreover, the materials disclosed in Haugh have little or no reflection efficiency when the material is imaged to form a reflection hologram. Thus, there continues to be a need for improved materials for use in preparing reflection holograms in general.